In general, there are available, as an operating mechanism of a switchgear, one using a hydraulic operating force for high power and one using a spring operating force for middle/small output power. The former is referred to as “hydraulic operating mechanism” and the latter as “spring operating mechanism”. In recent years, the advancement of miniaturization of an arc-extinguishing chamber of a gas circuit breaker which is a type of a switchgear allows fault current to be broken with a smaller operating force, so that application of the spring operating mechanism becomes popular. However, a gas circuit breaker of extra high-voltage class requires high-speed operating capability called “two-cycle cutoff” that is capability of achieving cutoff within a time length corresponding to two-cycle time periods of alternating current. A conventional spring operating mechanism typically has operating capability equivalent to about three-cycle operation, and it is not easy to realize the two-cycle cutoff capability due to poor responsiveness of a retention mechanism or retention control mechanism of a spring force.